treewide: kernel: drop redundant USB_EHCI_HCD=n config symbol

Commit e53ec043 ("kirkwood: move usb support to modules") has moved
this config symbol into generic configs, so it could be removed from
other configs.

Suggested-by: Aleksander Jan Bajkowski <A.Bajkowski@stud.elka.pw.edu.pl>
Signed-off-by: Petr Štetiar <ynezz@true.cz>